In numerous electrical-signal-driven printers, signals representative of the visual image on each page to be printed are supplied to the printer in numerical page order. If only one side of a sheet of recording medium is to be printed and only a single copy of each image, each signal is used when received to print the visual image as sheets are fed through the printer. If multiple copies are desired, the electrical signals are inserted when received into some form of a recirculating delay line and the delay line is recycled and read as many times as needed to print the number of copies desired. Printing on only one side of a recording medium is referred to hereinafter as simplex printing.
In many cases it is desirable to print visual images on both sides of the sheet of recording medium in numerical page order as in a book. Such printing is referred to hereinafter as duplex printing. If only one copy is needed, this can be accomplished by turning over each sheet immediately after it has been printed on one side and feeding it through the printer a second time to be printed with the visual image on the next following page.
Complications arise in making multiple copies of sheets with visual images on both sides in numerical page order. If each sheet is printed by first printing one side, turning the sheet over and then printing the other side, the time required to print a large number of copies of a lengthy job is prohibitive. On the other hand, if all copies of one page are made before turning the sheets over and copying the next page onto the other side, it becomes necessary to collate the sheets. If done by hand, collation of a large number of copies takes considerable time; and while mechanical collators are available for certain types of copying machines, they tend to be expensive and can only handle a limited number of copies.
As will be evident, analogous problems exist in duplex printing of multiple copies of visual images on a continuous web of a recording medium instead of discrete sheets. In such case, it is advantageous first to print one side of the web in its entirety, then the other side, to separate the web into individual sheets with a visual image printed on each side, and finally to collate the sheets as required.
For convenience, unless clearly indicated to the contrary, the term "recording medium" is used generically hereinafter to include both discrete sheets and continuous webs of any surface on which printing can be formed in accordance with the invention. Typically, the recording medium is paper. The term "page" is used to refer to one side of a sheet and the record formed thereon is referred to as an image. Ordinarily, in the practice of our invention, this image will be a visual image, that is, one discernible by the human eye; but our invention may be practiced using printed images that are not visible. The term "printing"is used in its broadest sense to mean the formation of images of any type whatsoever including, without limitation, pictures, charts, symbols, alpha-numeric text and the like.